An ion trap mass spectrometer is composed of a ring electrode and a pair of end cap electrodes opposing each other with the ring electrode therebetween. The inner surface of the ring electrode is formed hyperboloid-of-one-sheet-of-revolution and the inner surface of the end cap electrodes are formed hyperboloid-of-two-sheets-of-revolution. When appropriate RF voltages are applied on the ring electrode and the end cap electrodes, a quadrupole electric field is formed in the space (“ion trap space”) surrounded by the ring electrode and the end cap electrodes, whereby ions generated in the ion trap space or ions introduced from outside into the space are trapped and stored there.
After ions are trapped in the ion trap space, or while ions are stored there as explained above, various analyzing modes are possible by applying corresponding voltages to the end cap electrodes. FIGS. 5A-5C schematically illustrate some examples of frequency distribution of the RF voltage applied to the end cap electrodes for realizing various analyzing modes.
When, as shown in FIG. 5A, a sinusoidal signal having a certain frequency f1 which corresponds to the mass to charge ratio (m/z) of a certain ion is applied to the end cap electrodes, only the ions resonantly vibrate in the electric field and are ejected from the ion trap space, and other ions do not. When, as shown in FIG. 5B, a wide-band signal including a range of frequencies from f2 to f3 is applied to the end cap electrodes, ions having mass to charge ratio of a certain range corresponding to the frequency range vibrate simultaneously and are ejected from the ion trap space. Further, when, as shown in FIG. 5C, a wide-band signal devoid of a certain narrow range of frequencies from f4 to f5 (“notch”) is applied to the end cap electrodes, ions having the mass to charge ratios corresponding to the “notch” frequencies do not vibrate and remain in the ion trap space, while the other ions are ejected from it. Practically, the width of the notch f4-f5 is set appropriately according to the resolution of the ion trap mass spectrometer, so that the desired object ions can be selected and stored in the ion trap space.
When sample molecules or atoms are ionized, the following phenomenon occurs. Generally, atmospheric pressure chemical ionization (APCI) method and electrospray ionization (ESI) method are used for ionizing the sample in a liquid chromatograph/mass spectroscopy (LC/MS). These methods are categorized in soft ionizing methods in the sense that no dissociation of ions occurs. In these ionizing methods, besides a molecular ion M+ which is formed from a molecule minus an electron, various ions are generated such as a molecule plus H+ (proton), Na+ (sodium ion), K+ (potassium ion), NH4+ (ammonium ions) or a solvent ion, or a dehydrated ion which is a molecule ion minus a water molecule. Those ions are hereinafter referred to as “pseudo-molecular ions”. An example of a mass spectrum is shown in FIG. 6, in which dehydrated ion [M—H2O]+ and a molecular ion M+ are simultaneously generated. As seen in the mass spectrum of FIG. 6, peaks of impurities appear besides peaks of the object molecules.
Irrespective of ionizing methods, it often happens that plural electrical charges are added or deprived of a sample molecule, so that a multivalent ion is produced in the course of the ionization. An example of a mass spectrum including the peaks of multivalent ions is shown in FIG. 7, where peaks of undecavalent (11-valent) and further ions are omitted for visibility of the graph. In this case, also, peaks due to impurities appear.
When a component of an object sample is intended to be analyzed quantitatively with a mass spectrometer, it is necessary to measure not only the molecular ions of the component but also various ions derived from the molecule or atoms of the component. These ions have different mass to charge ratios, and, as shown in FIGS. 6 and 7, give rise to distinct peaks on the abscissa of the mass spectrum.
In conventional ion trap mass spectrometers, a wide-band signal having a notch of a certain width, as shown in FIG. 5C, is prepared for each ion derived from the component molecule that needs to be measured. The notch corresponds to the mass to charge ratio of the ion. Measurements are made one by one for each ion using the wide-band signal, and the results of the measurements are added to obtain the result of analysis.
Such a method is self-evidently complicated and inefficient. When an MS/MS analysis—in which selected ions (precursor ions) are dissociated in the ion trap space, and the mass spectrum of the dissociated fragment ions is obtained—is performed using the method, the amount of precursor ions becomes less and the amount of fragment ions also becomes less, so that an adequate mass spectrum can not be obtained. This deteriorates the detection sensitivity, S/N ratio and precision of the mass to charge ratio of the analysis.
In some ion trap mass spectrometers (ones made by Thermo Finnigan, San Jose, Calif., for example), the width of the notch is increased, or the difference of f4 and f5 in FIG. 5C is enlarged, and the range of mass to charge ratio is increased to cover all of the various ions to be measured. Thus the ion selections are performed simultaneously. In this method, for example, molecular ions M+ and proton-added ions MH+ can be selected simultaneously by enlarging the width of the notch by only 1 amu (if they are monovalent ions).
In order to simultaneously select molecular ions M+ and dehydrated ions (M—H2O)+ as shown in FIG. 8A, however, the notch width should be broadened by 18 amu than normal, as shown in FIG. 8B. When the notch width is thus broadened, it is probable that undesirable ions fall in the notch and remain in the ion trap space as shown in FIG. 8C. This produces chemical noises in the analysis.
In the case of multivalent ions as shown in FIG. 7, ions belonging to such a group have a wide variety of mass to charge ratios, and it is actually impossible anyway to select those ions simultaneously with the above method.
The present invention addresses the above problem. A primary object of the present invention is to provide an ion trap mass spectrometer that can select molecular ions and pseudo-molecular ions simultaneously, and that can certainly avoid remaining of unwanted ions. Another object of the present invention is to provide an ion trap mass spectrometer that can select multivalent ions having a variety of mass to charge ratios appropriately, and that can certainly avoid remaining of unwanted ions.